Perimetry is a very important and common test in clinical eye care, second only to the simple determination of visual acuity. It is used for examining the range and the sensitivity of a subject's visual field, e.g. in connection with diagnosis and treatment of glaucoma, for testing for neurological diseases, in mass screening etc.
In static threshold perimetry, the limit or threshold of a subject's light perception at a number of discrete locations in the visual field is determined. The test is conducted by means of a computerized perimeter, typically in the following way: the patient is seated in front of a perimeter and asked to look steadily at a centrally placed fixation target, e.g. on a screen or in a hemispherical bowl. Visual stimuli are presented successively with different intensities and at different locations. The patient is asked to press a response button every time he perceives a stimulus, whether close to or distant from the fixation target, whether faint or strong.
The test locations are usually tested in random order, a stimulus at a first test location being followed by a stimulus at another test location etc., and the next stimulus for the first test location being not presented until after several subsequent stimulus presentations.
After each presentation of a stimulus, the perimeter waits for a response during at most a predetermined time period. Stimuli which correspond to responses received within a predetermined response time window within the predetermined time period are classified as seen. Stimuli which correspond to responses received before and after the response time window are normally classified as unseen, as are stimuli for which no response at all is received. The response time window needs not to be the same for all test subjects. U.S. Pat. No. 5,381,195, by the inventors of the present application, discloses the use of a subject-adapted response time window.
However, it is well-known that patients sometimes press the button without having seen any stimulus at all, and they sometimes fail to press it despite having seen the stimulus. These kinds of responses are called false positive responses and false negative responses, respectively. It goes without saying that they affect the accuracy of the threshold determination.
The frequency of false positive responses may be examined by false positive catch trials. The perimeter then acts as when displaying a stimulus but without showing one, and registers whether the patient responds or not. The frequency of false negative responses may be examined by false negative catch trials. A strong, supraliminal stimulus is presented at a point where the threshold has already been measured, and the perimeter registers whether the patient responds or not. Often 20-30 catch trials are presented during a test.
There are different strategies for selecting test locations and intensities of the stimuli presented at these test locations in order to establish a patient's threshold for perception of light. In one common method, a stimulus is shown having an intensity close to an expected threshold value at each test location concerned. If the patient does not respond to the stimulus, the intensity of the subsequently presented stimuli is thereafter increased stepwise until a response is received from the patient, i.e. until a stimulus is seen. The first intensity level at which a response is received can be defined as the threshold of the test location concerned. The precision of the test can be increased by reversing the test process when the first response is received, and by continuing it in smaller steps with decreasing intensities until the first unseen stimulus is encountered. The threshold can then be defined as the average intensity level of the last seen stimulus and the first unseen stimulus. If, on the other hand, the patient responds to the first stimulus, the intensity is decreased stepwise until no response is received, whereupon the test procedure is reversed.
The above method of presenting series of visual stimuli with alternately increasing and decreasing intensities is called the staircase method.
In the staircase method, the intensity steps between stimuli of increasing/decreasing intensities are often constant, at least between reversals. A variant hereof is the Robbins-Monroe method where the steps between successive stimuli are gradually decreased.
Another method for determining threshold values is the Modified Binary Search (MOBS), according to which a stimulus with a selected intensity close to the expected threshold value of the subject is presented. If the stimulus is seen, its intensity value is regarded as the upper threshold boundary and, if it is not seen, as the lower threshold boundary. The intensity range is then divided into a series of increasingly smaller half-intervals until the upper and lower treshold boundaries are within a predefined range.
There are also other test strategies. However, they all classify each stimulus as either seen or unseen, using this classification as a basis for the selection of the next intensity of the test location and as a basis for determining the threshold value.
When the threshold values for all the test locations in the visual field have been determined, they are often compared with normal, previously determined threshold values for subjects of the same age to establish whether there are any deviations from normal, or with previous values for the same eye of the subject to establish whether a disease under observation has progressed or receded.
A problem in this context is that the measurement variations are considerable and that there is a high proportion of false positive and false negative answers. One reason for this may be that the test is very tiring. As described above, several stimuli of different intensities are presented at each test location on the screen. Since 50-100 test points are usually examined, one test consists of several hundred stimulus presentations. Typically, the time required for a complete static threshold perimetry test is about 10-20 minutes per eye.
The above-mentioned measurement variation makes it difficult to follow the development of a disease, because the errors of the calculated threshold values will be large. Furthermore, it makes it difficult to precisely define the limits between a normal visual field and an abnormal visual field. If the limits of what is considered as a normal visual field are too narrowly set, some patients will unnecessarily be further examined and/or treated. On the other hand, if the limits are too wide, the abnormal visual fields of some patients will be interpreted as normal, and these patients may not be further examined and/or treated.
One object of the present invention is, therefore, to provide a method and an apparatus which reduce the measurement variation and result in more precise threshold values.